Hematocrit of the human cranial blood pool.

• Since the hematocrit of arteriolar and capillary blood is substantially lower than that found in larger vessels, the mean hematocrit of total organs will be lower than the large vessel hematocrit, particularly in organs, such as human brain, that contain very rich capillary beds. This has been demonstrated in many tissues.-~' It is the purpose of this article to describe a basic technique for determining safely and simply the mean hematocrit of the human cranial blood pool. The method utilizes the separate and sequential intravenous administration of gamma-labeled red cells and plasma. By means of an externally placed collimation system, the red cell and plasma volumes in the cranial portion of the head are measured. The region referred to here includes all tissues rostral to the floor of the cranial cavity. These cranial volumes are compared with red cell and plasma volumes in venous blood measured in specimens drawn simultaneously from an antecubital vein midway during the cranial counting periods.